Organic by-products and residues produced from many sources require active management which can range from simple to complex in nature. On the simple end of the scale, agricultural manure and non-commercial forestry residue can be reduced in size and incorporated into the soil as conditioning agents. Much more complexity and cost is associated with the management of organic residues produced in commercial or residential activities such as meal preparation. In general, the closer the organic material comes to the consumer, the more complex and costly the management of organic residue becomes.
The highest standard of care for organic residues is generally associated with management of residential organic waste. The majority of solid organic waste including that from residential, commercial and industrial sources has traditionally been managed by placement in solid waste landfills designed to limit the infiltration of moisture, and limit anaerobic degradation and biogas production. The objective of this management practice has been to minimize environmental impact by leaving the waste materials in place indefinitely. The permanent entombment of waste in landfills has become an increasingly undesirable waste management practice for many reasons, real or perceived, including the following:                Risk of eventual liner failure and/or groundwater contamination        Waste of valuable resources        Potential for gas migration and explosion hazards        Global warming impact        NIMBY (not in my backyard) factor in getting new facilities approved        Lack of land space in some areas        Social pressure to find alternatives to landfill        The permanency of waste placed in a landfill        
Nevertheless, society continues to generate organic waste in ever greater quantities, and despite increased interest in “diversion from landfill” initiatives, the economically efficient management of solid waste remains a significant challenge for most communities. During the past twenty years, the diversion of source separated recyclable materials from residential, industrial and commercial generators has become an established component of most communities' waste management systems. The recovery of aluminum, steel, glass, fibre and plastic is now normal practice in many communities and provides valuable raw materials for a wide range of industrial uses. The single largest component of the solid waste stream that has yet to be economically diverted from landfill on a large scale is organic waste. While leaf and yard waste is routinely collected and composted in many communities, household organic waste (primarily food) is not.
Many attempts have been made to develop large scale economically viable in-vessel processing technologies to manage this waste stream but, as yet, these have met with minimal success for a number of reasons including:                The heterogeneity of residential organic waste        The cost to construct and operate        The failure to create a technology that achieves the economies of scale of the landfill and thereby provides an economically viable alternative to landfill. Most technology to date has tried to prepare and process solid and semi-solid organic waste through pipes and vessels using process engineering principles and techniques.        Poor odour control        Difficulty in dealing with the sheer volume of material generated in a community on a daily basis and that requires immediate “treatment”.        
Incineration with energy recovery has also been used widely, particularly in regions where space is at a premium. Like landfill, incineration is viewed as a “disposal” option (particularly in North America) without resource recovery (although energy is often recovered), is expensive, and because of air emissions, is perceived negatively by many.
In recent years, public and political pressure has led to initiatives to divert the valuable solid and semi-solid organic waste stream away from disposal, whether it be landfill or incineration, and towards compost (aerobic processing) and biogas (anaerobic) production plants. One of the great advantages of the landfill and its bioreactor hybrid is that it is relatively cheap to build and operate compared to the competing technologies.
Within a solid waste landfill both aerobic and anaerobic processes take place. Landfills continue to digest waste and produce landfill biogas gas for years or decades depending on many factors including the size of the fill, nature of the waste, moisture content, site design and local climate. Since the majority of material found in a landfill typically consists of inert materials intermingled with organics, no compost product of any value can be economically recovered. But the landfill tolerates any manner of foreign objects and contamination without adverse effect on the biological processes.
Considerable work is currently being carried out to support the operation of landfills as bioreactors. The bioreactor concept is focused on the accelerated degradation of the organic materials contained in a landfill in order to more quickly stabilize the waste mass, to increase the effective yield of the airspace by reducing the volume of waste through organic degradation and to render the waste left in the landfill relatively inert following landfill closure. However, the bioreactor is a landfill because waste remains in place following closure.
Aerobic composting is normally carried out either in open air windrows (e.g. leaf and yard waste) or in simple channel reactors inside a building (food waste) followed by final curing in windrows. It is less complex and less costly per unit weight processed than anaerobic digestion. It is also relatively tolerant of foreign matter in the feedstock. However, because the process is comparatively slow, it requires a substantial amount of space per unit weight of material treated compared to anaerobic digestion, and no energy recovery is possible.
In-vessel anaerobic digestion is very efficient at killing pathogens, is capable of producing valuable energy, is relatively rapid and uses less time and energy per unit weight processed than aerobic composting. On the other hand it tends to produce very powerful odours which need to be contained and treated. This in turn means the process has to be conducted in an enclosed building and in sealed vessels with elaborate odour control systems. Because of the greater degree of technical sophistication, the process is costly both to build and to operate. Also, relatively minor amounts of foreign matter in the feed can damage the plant and restrict its throughput, further increasing costs.
However, in-vessel anaerobic digestion is a proven technology in the treatment of organic materials including wastes such as sewage, animal waste and food processing waste. Typically, the technology is applied in a process plant environment where the organic feedstock is moved through a processing plant, usually as a liquid but sometimes as a solid and, because of this, a high degree of control of process parameters such as feedstock quality and chemical composition, viscosity, temperature, Carbon/Nitrogen (C:N) ratio, pH, reactor retention times, flow rates etc. is necessary to provide effective treatment. In this type of operating environment, it is critical that the control parameters are maintained within narrow operating ranges. There are many technologies commercially available to anaerobically treat organic liquid or solid materials within the plant processing model where the material is moved through a series of pipes and vessels in order to complete the process. Because the material is being processed through pumps, pipes and vessels of relatively low capacities, control of process parameters including retention times are critical for treatment and economic success. However, solid organic waste as it is generated from municipal source separated organic waste collection programs for example, can be a very heterogeneous feedstock, and consequently difficult to economically process through this type of plant.
A unique feature of this invention is that it eliminates the movement of the organic feedstock through pipes and vessels (with vessel retention times measured in the “hours-to-days” operating range) and instead places the organic feedstock in a large reusable engineered containment structure where it remains through a sequence of anaerobic-aerobic treatment environments until treatment is complete. In this invention, the organic material remains stationary, and the treatment process moves. As a result the retention time in the reactor is measured in the “months-to-years” range and thereby eliminates one of the key production challenges of the processing plant “pipe and vessel” approach. This invention incorporates the economies of scale and proven anaerobic degradation processes that occur in the landfill/bioreactor with the continuous treatment capability of the anaerobic and aerobic processing plant approach. With this invention, the constructed containment capacity is not sacrificed as it is in the landfill/bioreactor, but is available for repeated reuse as it is in the processing plant environment.
In the patent literature Ham et al., U.S. Pat. No. 5,984,580, issued Nov. 16, 1999 describes a method of improving conventional landcharging techniques wherein the waste to be landcharged is comminuted to yield homogeneously-sized waste particles, mixed with liquid and then placed into a sanitary containment site with a leachate and gas collection system such that moisture is distributed uniformly throughout the waste mass. Leachate is recirculated to accelerate the anaerobic degradation processes, thereby maximizing the production of landfill gasses and accelerating the stabilization of the site within years instead of decades to reduce environmental impact and enhance landfill gas production. However, no useable compost is recovered from the bioreactor and its constructed containment structure cannot be reused since the treated waste remains in place.
Hudgins et al. U.S. Pat. No. 6,364,572, issued Apr. 2, 2002, describes an aerobic landfill bioreactor whereby decomposing municipal solid waste in the landfill is aerobically degraded using various techniques to inject air and by recirculating the leachate to achieve certain moisture levels and temperatures. An objective of this invention is to prevent the generation of landfill gas and so energy recovery is not possible.
Layton et al., U.S. Pat. No. 6,481,929, issued Nov. 19, 2002 also describes a method for the accelerated and enhanced aerobic bioreduction of municipal solid waste within the landfill by the use of a novel moisture and air injection system followed by the optional excavation of the landfill cell materials for separation into usable compost materials, and reclamation of recyclable plastic, metal and glass. Because this process does not allow anaerobic digestion to occur, landfill gas recovery is not contemplated, nor is the continuous reuse of the constructed containment structure that is the landfill considered.
Pliny Fisk, U.S. Pat. No. 4,053,394 issued Oct. 7, 1977 describes a process to treat sewage that begins with an anaerobic stage followed by an aerobic stage and final curing. While the patent incorporates an anaerobic-aerobic treatment sequence, it is provided through a complex material handling and processing plant, does not contemplate the processing of solid material of any type and does not incorporate energy recovery.
Hater et al., U.S. Pat. No. 6,283,676 issued Sep. 4, 2001 describes a sequential aerobic-anaerobic solid waste landfill operation whereby as the landfill is constructed in vertical lifts, systems for the removal and re-circulation of liquids and gases are installed to accelerate anaerobic and/or aerobic degradation of the municipal solid waste in order to increase landfill capacity. This patent does not provide for the removal of stabilized materials and the reuse of the containment structure.
An objective of this invention is to efficiently convert solid and semi-solid organic material, potentially supplemented by liquid organic material and/or water into two valuable products: energy and soil conditioners by combining several proven unit operations including feedstock preparation, anaerobic digestion, aerobic composting, biogas recovery and conversion to power, excavation and product finishing.